Nitrobenzene is an important intermediate product of the chemical industry which is required in particular for the preparation of aniline and therefore also for the preparation of di- and polyisocyanates of the diphenylmethane series and the polyurethanes based thereon. The nitration of benzene with nitric acid to give a crude nitrobenzene has already been the subject of numerous publications and patent applications. The present current processes substantially correspond to the concept of adiabatic nitration of benzene by a mixture of sulfuric and nitric acid (so-called mixed acid). Such a process was claimed for the first time in U.S. Pat. No. 2,456,999 and is described in present-day embodiments, for example, in EP 0 436 443 B1, EP 0 771 783 B1 and U.S. Pat. No. 6,562,247 B2. The processes with an adiabatic reaction procedure are distinguished in particular in that no technical measures are taken to supply heat to or remove heat from the reaction mixture.
Isothermal processes for the nitration of benzene with mixed acid are also described, such as are described, for example, in EP 0 156 199 B1.
Processes for the nitration of benzene which manage without the use of sulfuric acid are also known. These are described, for example, in U.S. Pat. No. 2,739,174 or U.S. Pat. No. 3,780,116.
Gas phase processes for the nitration of benzene with nitric acid or nitrogen oxides are in principle also possible, but the yields which can thereby be achieved are still low (EP 0 078 247 B1, EP 0 552 130 B1).
All these processes have the common feature that the reaction product first formed is a crude nitrobenzene which comprises nitric acid and—if the nitration has been carried out with mixed acid—sulfuric acid, as well as dinitrobenzene and nitrated oxidation products of the benzene, in particular nitrated phenols (nitrophenols), as organic impurities it also comprises organic compounds which have formed from the compounds which the benzene employed contained as impurities (WI) 2008/148608 A1). The crude nitrobenzene moreover also comprises metal salts, which can be present in dissolved form in the acid residues or in the crude nitrobenzene (DE 10 2007 059 513 A1).
Numerous investigations in the past have been targeted at improving the quality of the crude nitrobenzene and thus at increasing the yield with respect to benzene and nitric acid. Thanks to these developments, the present-day adiabatic liquid phase processes have developed to the extent that they all succeed in preparing a crude nitrobenzene which has a low content of by-products, that is to say in general comprises only between 100 ppm and 300 ppm of dinitrobenzene and between 1,500 ppm and 2,500 ppm of nitrophenols, it being possible for picric acid to assume a content of from 10% to 50% of the nitrophenols.
The crude nitrobenzene still contains as impurities water, benzene and nitrophenols and dinitrobenzene and—if the nitration has been carried out with mixed acid—sulfuric acid. These impurities are undesirable, since in subsequent processes where nitrobenzene is employed, such as, for example, the preparation of aniline, they can adversely influence these. Suitable work-up processes which comprise washing and distillation steps are described e.g. in U.S. Pat. No. 6,288,289 B1. EP 1 593 654 A1, EP 1 816 117 B1 and WO 2011/021057 A1.
EP 1 816 117 B1 describes the work-up of the crude nitrobenzene in an acid wash, an alkaline wash with aqueous sodium hydroxide solution, a neutral wash and a final purification by distillation. In principle bases other than sodium hydroxide solution can of course also be used, such as, for example, aqueous sodium carbonate solution or aqueous ammonia solution (WO 2011/082 977 A1) or potassium hydroxide or ammonia (DE 60 113 579 T2).
The work-up of the alkaline waste water from the alkaline wash can be carried out e.g. by so-called “thermal pressure decomposition” (also TPD in the following). The fundamental process of TPD for treatment of waste waters comprising aromatic nitro compounds is described in the following patents:
EP 0 005 293 B1 describes a process for working up waste waters comprising nitro-hydroxyaromatics, wherein the waste waters are treated under a pressure of from 50 bar to 250 bar at a temperature of from 150° C. to 500° C. with exclusion of air and oxygen.
EP 0 503 387 B1 has described a similar process, which, however, is characterized in that the said alkaline waste water is worked up by addition of nitric acid and subsequent treatment in temperature ranges of from 180° C. to 350° C. under a pressure range of from 40 bar to 250 bar. However, both processes have considerable disadvantages:
EP 0 005 203 B1 does not describe the removal of organic hydrocarbons such as benzene or nitrobenzene which are produced in an adiabatic nitration process corresponding to the state of the art. The purification of the waste water according to the teaching of EP 0 005 203 B1 is therefore either insufficient, or the consumption of sodium hydroxide solution in the TPD is very high.
In EP 0 503 387 B1 complete decomposition of nitrobenzene does not succeed, so that a further treatment of the waste water is necessary. The nitrobenzene contained in the waste water moreover is decomposed in the TPD and therefore decreases the yield achieved. The presence of nitric acid in the TPD required according to the teaching of EP 0 503 387 B1 furthermore drives up the process costs in two respects: on the one hand due to the consumption of nitric acid and on the other hand due to the high material stresses as a result of the corrosiveness of the nitric acid and the high investment costs associated with that, for example for a corrosion-resistant tubular reactor lined with titanium. An additional disadvantage, which is not described, is also the need for the alkaline waste water initially having to be neutralized (e.g. with additional nitric acid), before this can be converted into an acid pH range by addition of the nitric acid. A relatively large amount of acid is thus necessary.
EP 1 593 654 A1 describes a process for working up alkaline waste waters which are formed during the washing of crude nitrobenzene, wherein the crude nitrobenzene is prepared by adiabatic nitration of benzene with nitrating acid and is then washed in an acid wash and thereafter in an alkaline wash, wherein an alkaline waste water comprising benzene in concentrations of from 100 ppm to 3,000 ppm and nitrobenzene in concentrations of from 1,000 ppm to 10,000 ppm is obtained, wherein benzene and/or nitrobenzene present in undissolved form are then separated out of the alkaline waste water, and the residual benzene and/or nitrobenzene is then optionally removed from the alkaline waste water by stripping, and the alkaline waste water is then heated to temperatures of from 150° C. to 500° C. under increased pressure, with exclusion of oxygen. The waste water treated in such a way can therefore be fed directly to a biological treatment plant without dilution. However, by decomposition of nitro compounds in aqueous waste waters according to the state of the art, ammonia is formed, which has an adverse effect in biological treatment plants (see also the following paragraph regarding WO 2012/025393 A1).
WO 2012/025393 A1 describes the work-up of waste waters which are obtained in the purification of crude aromatic nitro compounds after the nitration of aromatic compounds, and in particular deals with the problems of removal of ammonia, which is formed during the thermal decomposition of nitro compounds. The process described provides the following steps; (a) one-stage or multi-stage washing of the crude aromatic nitro compound to obtain at least one organic phase and at least one aqueous phase, and separating off of the aqueous phase or the aqueous phases, wherein step (a) includes the addition of a base which differs from ammonia, and then (b) optionally removal of organic constituents from at least a part of the aqueous phase or aqueous phases obtained in step (a) by stripping, preferably with steam, subsequently (c) removal of organic compounds from at least a part of the aqueous phase or aqueous phases resulting from step (a) or, respectively, step (b) by thermal and/or oxidative degradation, subsequently (d) depletion of ammonia from at least a part of the aqueous phase or aqueous phases resulting from step (c) by distillation, and subsequently (e) optionally feeding of at least a part of the aqueous phase or aqueous phases resulting from step (d) to a biological waste water treatment. In particular, the ammonia content in the waste water in step (d), which is produced in the nitration of aromatic compounds and subsequent removal of organic constituents, should be reduced. In this connection WO 2012/025393 A1 refers to free ammonia and not dissolved ammonium ions in the waste water after the TPD, which is fed to a biological waste water treatment. From the carbon dioxide present in the alkaline waste water and the ammonia, in fact, ammonium carbonate always also forms to a certain extent, which as a result of its salt character cannot be removed by stripping like ammonia.
There was therefore a need for an advanced process for working up alkaline waste waters which are formed during washing of crude nitrobenzene prepared by adiabatic nitration of benzene. In particular, it should be possible to use the processes proven per se for thermal pressure decomposition (TPD) without the content of ammonia and ammonium ions in the waste water thereby formed presenting problems in the further work-up, for example in a biological treatment plant.